js实现控制textarea输入字符串的个数,鼠标按下抬起判断输入字符数

<table>
<tr>
  <td width="150">短信内容：</td>
  <td>
   <textarea name="message" cols="96" rows="5" onKeyDown="textCounter(message,remLen,65);"
     onKeyUp="textCounter(message,remLen,65);"></textarea>
  <td>
</tr>
<tr><td></td>
  <td>您还可以输入:<input name="remLen" type="text" value="65" size="5" readonly="readonly">个字符,每条短信最大允许输入<strong>65</strong>个字符</td>
</tr>
</table>

<script>
   function textCounter(field, countfield, maxlimit) {
      if (field.value.length > maxlimit)
        field.value = field.value.substring(0, maxlimit);
      else
        countfield.value = maxlimit - field.value.length;
   }
</script>